Video communication has evolved over the years from a simple video telephone concept to a sophisticated network for allowing multiple parties to enter into a video conference. A number of factors have prevented total success of such prior video systems, including public acceptance, excessive cost, system complexity and inadequate video quality. Although these factors can be manipulated somewhat to provide an improved system for video communications, inherent constraints such as standardized video formats and presently existing communications systems minimize the design flexibility which may be utilized in achieving a feasible system.
Video communications can be utilized for a number of applications. In some applications, only transmission of a hard copy is required, such as a picture or graphics representation. Such hard copy transmission is normally accomplished by such techniques as facsimile transmission to enable transmission of such things as x-rays, flow charts, and the like. However, for a full video conferencing system wherein individuals desire both an audible communication path and a real time visual communication path, it is necessary to provide full motion color video. The transmission of full motion color video normally requires a much greater bandwidth than the transmission of facsimile data. This is due to the fact that the video is transmitted in "real time" to allow an interactive conference.
Non real time video systems have been previously developed for teleconferencing which operate on lower bandwidth communication links. One type of non real time system is often referred to as the slow scan video system which is programmed to send new pictures at regular intervals, whether the image is changed or not. In this type of system, movement is prohibited in order to provide a relatively clear picture. An alternate to the slow scan system is the freeze-frame system which records a clear picture of the speaker and transmits this clear picture to the remote terminals in the conference at regular intervals. However, the slow scan and freeze-frame systems are not real time and the parties viewing the conference are only provided a sequence of stepped poses for the speaker.
Full motion video, heretofore the most preferred for video conferencing, is accomplished by a number of techniques. The most effective full motion video conferencing systems transmit over 1.544-megabits/s T1 telephone lines. Although the picture quality on these high bandwidth systems is high, so are the operational costs per hour for nationwide point-to-point connections.
To decrease the cost, data compressed video conferencing systems have been developed which operate on a 56-kb/s packet-switched network. Data compression is required to operate on these 56-kb/s systems, since direct digitization of standard NTSC broadcast video color signals requires approximately 80 megabits per second, far beyond the capacity of most transmission lines. To transmit full-motion color at lower bandwidths, the digital signal must be compressed by the removal of redundant information.
Two main data compression approaches have been heretofore developed, namely interframe coding and intraframe coding. In interframe coding, successive video frames are compared, pixel by pixel, and only changed values are transmitted. In intraframe coding, values for entire blocks of pixels within a frame are transmitted as mathematical transforms. These techniques are useful for transmitting at 1.544-Mb/s over T1 lines. However, transmission at 56-kb/s requires further data compression. This is accomplished by squeezing out data on luminescence, hue, resolution and scan rate. A cosine transform is utilized to compress the data efficiently, with the negative result being breakup of the picture into blocks of pixels when the transform needs time for each recalculation and the system has too many bits to send. Other prior 56-kb/s systems use a binary algorithm that degrades by losing resolution when overwhelmed by too much motion.
The disadvantages to the 56-kb/s systems and similar digital systems are that they first require the availability of a digital network and, secondly, they require a relatively expensive codec to interface between the video terminal and the network. These systems are seldom available or financially feasible for local or intrafacility use.
In prior video conferencing systems, another major disadvantage has been the initial cost of the remote terminals which are, at present, dedicated to video conferences. The longer a video terminal remains idle, the higher the cost-per-hour. Therefore, it would be desirous to integrate the remote video terminal with other functions to lower the effective cost per hour of the video conferencing feature.
In view of the above disadvantages of prior video conferencing systems, there exists a need for a full motion, color video conferencing network that overcomes the deficiencies of the present systems and is more adapted to economical local and intrafacility use.